Since TCP/IP is the most used network protocol nowadays, let’s make a correlation between the TCP/IP protocol and the OSI reference model. This will probably help you to better understand both the OSI reference model and the TCP/IP protocol.

As we’ve just seen, the OSI reference model has seven layers. TCP/IP, on the other hand, has only four, so some layers from the TCP/IP protocol represents more than one layer from the OSI model.

On Figure 3 you can see a correlation between the OSI reference model and the TCP/IP protocol.

Figure 3: TCP/IP architecture.

The idea behind TCP/IP is exactly the same we explained about the OSI reference model: when transmitting data, programs talk to the Application layer, which in turn talks to the Transport layer, which then talks to the Internet layer, which then talks to the Network Interface layer, which sends frames over the transmission media (cable, air, etc).

As we mentioned earlier, TCP/IP isn’t the name of a specific protocol, but the name of a protocol stack, i.e. a set of protocols. Each individual protocol used on the TCP/IP stack works on a different layer. For example, TCP is a protocol that works on the Transport layer, while IP is a protocol that works on the Internet layer.

It is possible to have more than one protocol on each layer. They won’t conflict with each other because they are used for different tasks. For example, when you send out e-mails, your e-mail program talks to the SMTP protocol located on the Application layer. Then this protocol, after processing the e-mails received from your e-mail program, sends them to the layer below, Transport. There data will be processed by the TCP protocol. When you browse the web, your web browser will also talk to the Application layer, but this time using a different protocol, HTTP, as this is the protocol in charge of processing web browsing.

Here is a brief explanation of each TCP/IP layer:

• Application: As we mentioned, programs talk to this layer. Several different protocols can be used on this layer, depending on the program you are using. The most common are HTTP (for web browsing), SMTP (for sending e-mails), POP3 (for receiving e-mails) and FTP (for transferring files).
• Transport: Everything we said about the Transport layer from the OSI reference model is valid for the TCP/IP Transport layer. Two different protocols can be used on this layer, TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). The first uses the acknowledge scheme explained before, while UDP doesn’t. TCP is used for transmitting user data (like web browsing and e-mails) while UDP is more commonly used for transmitting control data.
• Internet: Everything we said about the Network layer from the OSI reference model is valid for the TCP/IP Internet layer. Several protocols can be used on this layer and the most common one is the IP protocol.
• Network Interface: This layer is in charge of sending data to the transmission media. What is inside this layer will depend on the kind of network you have. If you are using an Ethernet network (the most common network type) you will find the three Ethernet layers (LLC, MAC and Physical – LLC stands for Logic Link Control and MAC stands for Media Access Control) inside this TCP/IP layer. The Physical layer from Ethernet networks corresponds to the Physical layer from the OSI model, while the other two layers (LLC and MAC) correspond to the Data Link layer from the OSI model.

We’ll stop here. To learn more about the TCP/IP protocol please read our How TCP/IP Protocol Works tutorial.